The invention also relates to a method of manufacturing such a display unit.
A display unit here means any unit for generating stationary or moving images from electric signals, said images being constituted by typographical characters and/or by line drawings and, possibly, having various shades or values of grey.
Although a usual television screen constitutes such a display unit, the present invention does not relate to such a TV screen, since the optically active layer, i.e. its layer which generates the image, of a TV screen is not usually constituted by a fluid.
In contrast, plasma display units and liquid crystal display units constitute known types of display unit which include layers of optically active fluid. Such layers are disposed between two plates at least one of which, namely, the front plate, is transparent and has optical characteristics controlled by electric signals.
In the case of liquid crystal display units to which the invention advantageously applies, the front and back plates carry row electrodes and column electrodes constituted by transparent conductive strips. The active layer of liquid crystal must have a micrometric thickness, i.e. a thickness which lies between 2 microns and 100 microns, e.g. of the order of 10 microns. This thickness must be perfectly constant over the whole of the useful portion of the layer, the tolerance being of the order of 7% in the case of a nematic liquid crystal whose birefringence is electrically controlled and with which it is required to obtain 16 shades of grey.
It is difficult to keep to such a tolerance when the useful surface of the display unit is not small, e.g. when this surface is 15 cm wide and 9 cm high. To do so, it is known to produce both the plates in the form of two perfectly plane glass plates which are very rigid and are fixed by their edges to the frame of the display unit. The necessary rigidity is obtained by choosing very thick plates, e.g. 12 mm thick, whose planeness results from a smoothing operation. Insulating shims are disposed between the two plates in the neighbourhood of their edges to maintain the required gap between them. A peripheral sealing strip makes it possible to form a sealed chamber between these two plates.
The cost price of plates for known display units is very high and restricts the field of application of these display units.
That is why it has been proposed to use relatively thin, resilient plates and to maintain a suitable distance between these plates by spacers inserted between them and spaced out over the whole useful surface of the display unit. Such a disposition is described in document U.S. Pat. No. 4,130,048 (Crossland).
The cost price of display units thus produced is however still high in particular because it is difficult to make the numerous electric connections between the control circuits and the transparent electrodes.
The present invention aims to provide a two-dimensional display unit which contains an electrically controlled fluid layer and in which it is very easy to make the electric connections, to keep the thickness of the optically sensitive layer very constant without using rigid glass plates and to assemble the two plates.